Myeloperoxidase (MPO) and elastase, restricted to azurophil granules of neutrophils, as well as lactoferrin, restricted to specific granules of neutrophils, were determined in plasma and serum from patients with acute myeloid leukaemia (AML). Highly sensitive radio immuno assays were developed for detection of these proteins. Serum MPO was increased in 12/35 and decreased in 2/35 patients without correlation to WBC or neutrophil counts; these levels may reflect an abnormal production by leukaemic blasts or ineffective granulopoiesis in the bone marrow. Serum elastase was increased in 6/22 patients. Serum lactoferrin was decreased in 12/25 patients without correlation to neutrophil counts probably reflecting abnormal production. Serum elastase and MPO showed a covariation in chronic myeloid leukaemia but not in AML; the latter finding may indicate that the synthesis of these two proteins is not synchronized in AML-cells. Sequential studies of patients with AML demonstrated fluctuations of serum MPO and lactoferrin during remission most likely because of chemotherapeutic pertubation. Although a limited number of patients has been studied it is suggested that serum lactoferrin may be of help for prediction of relapse in AML.